An unknown quantity
Definition 1 of 1
Pro Tip 1/3Newcomer Unpredictability
Use it to describe a new member whose effects or actions are unpredictable due to lack of information. 
Our team's new recruit is an unknown quantity; we hope he'll perform well.
Pro Tip 2/3Variable Situations
It can refer to a situation where outcomes are unpredictable because variables are not yet understood. 
With so many variables, the project's success is an unknown quantity.
Pro Tip 3/3Mystery Factor
Highlight a specific characteristic or factor that is mysterious and yet to be discovered. 
In this equation, the role of the new enzyme remains an unknown quantity.